Willows (Salix spp.) are of commercial importance as they provide renewable and sustainable biomass for bioenergy. They grow fast, produce high yields with low fertilizer inputs and easily re-grow after being coppiced (cut back to their base). Coppicing response is of fundamental importance as it enables willows to be grown in three year harvesting cycles, affects vigour and yield, stem and crown architecture and the ratio of bark to wood in the stem. Despite its importance the genetic regulation of coppicing response is little understood. The team used knowledge and methodologies from the model plant species Arabidopsis to identify SxMAX4 as the first coppicing response gene known to date. The study is published today in the Plant Biotechnology Journal.

Although cultivated since the Roman times, and still used for baskets and cricket bats, willows have been subject to only minimal domestication. Luckily, the genus shows huge genetic diversity, from massive trees to small bushes, and there is great potential for crop improvement through breeding. However, improving willows is challenging. Many target traits, including coppicing response, are complex and a good understanding of the genetic basis of developmental processes is required in order to develop improved willow varieties. Studying development in willows is difficult due to their large size, perennial growth cycle and the effort required to assess phenotypes in the field. Also gene function assays that are routinely used in model organisms are not yet established in willows.

Arabidopsis is undoubtedly the best characterized and widely-studied model plant today and there is great interest in transferring knowledge from Arabidopsis to species of commercial importance, such as willows, that are much harder to study. The current findings demonstrate the great results that can be achieved from exploitation of the vast body of knowledge in this model, and of combining plant molecular biology with genetic mapping approaches.

Dr Angela Karp of Rothamsted Research and lead scientist in this study said: “It was known that re-growth of shoots from the cut base after coppicing results through the activation of dormant buds in the coppiced base, and we set up genetic crosses to study this, but the developmental control of this process is very difficult to investigate in willow. In Arabidopsis, many genes are known to control bud activation through a network of interacting plant hormones. We chose to focus on the More AXillary growth (MAX) genes as these are known to have key effects on branching in Arabidopsis. We isolated the genes from willow and Ottoline Leyser’s group introduced them into Arabidopsis plants that lacked functional copies of the relevant MAX genes. We were then able to see the effects of the willow MAX genes in Arabidopsis using a standard branching assay”.

Dr Karp added: “It was really exciting to find that one of the MAX genes, SxMAX4, which affected branching in the Arabidopsis assay, was associated with differences in shoot re-sprouting in willow, and is located in exactly the area in the willow genome that is responsible for regulating this trait. This is the first time a gene has been shown to influence coppicing response and should enable the breeding of plants with desired stem numbers not only in willow but other coppiced trees”.

Professor Ottoline Leyser of the Sainsbury Laboratory said: “This has been a very exciting collaboration for me. I expected that our Arabidopsis work would be useful for understanding trees such as willow, but I did not anticipate quite how similar bud activation in these two species would be. The approach we have used to test willow genes in Arabidopsis could be widely useful for assessing functional genetic diversity in slow-growing species.”